Neutralization of high gain amplifiers



p 1952 N G. D. WOFFORD 2,610,261

NEUTRALIZATION OF HIGH GAIN AMPLIFIERS Filed May 23, 1947 GEORGE D. WOFFORD Patented Sept. 9, 1952 UNITED STATES rArsNroFFics George D. Wofio id, Towson, .Md., assignor to Bendix Aviation, Corporation, Towson, MIL, a

corporation of pelaware 1 Application-Ma .123, 1947, Serial No. 749,986

' mass. (01, "1719471);

This invention is directed to vacuum. tube amplifiers. More specifically, it is directed to the prevention of excessive regeneration in high gain amplifiers incorporating tetrode or pentode tubes.

The problem of reducing regeneration in high gain amplifiers, particularly at high radio frequencies has in the past been attacked from several angles. The development of multielectrode vacuum tubes with their low grid-plate capacity appeared at one time to have solved the problem. The recent development of tubes having extremely high transconductance and consequent high gain per stage, however, has brought the problem to the fore again. Grid-plate capacity is not the only offender, however, for the circuit and wiring capacities are often sufficient to produce excessive regeneration in modern amplifiers. This excessive regeneration may manifest itself by oscillation of the amplifier, but even in installations where the stray capacities are insufficient to sustain oscillation the presence of regeneration may have an adverse effect on the selectivity characteristic.

It is an object of this invention to provide a high gain amplifier with a minimum of regeneration.

This object is attained by the use of an impeder associated with the screen electrode of an amplifier tube in such a manner that a neutralizing voltage is applied to the said screen.

The above and further objects and advantages of the instant invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing wherein Fig. 1 is the schematic dia gram of an amplifier incorporating the instant invention, and Fig. 2 is a modified version of the Fig. 1 embodiment.

Referring now to Fig. 1, there is illustrated a pentode vacuum tube l connected as an ampliher having input terminals H connected to the control grid and cathode, and a suppressor grid tied directly to the cathode and the negative terminal of source of voltage 12. The screen of the tube I 0 is connected to the positive terminal of the source I2 through a resistor l3 and to the anode of the tube through a tuned cir cuit I4.

In operation, this arrangement is a conventional tuned pentode amplifier, with the addition of the resistor I 3 and the elimination of the usual screen bypass condenser. The input impedance seen at the terminals II will normally exhibit a negative resistive component at a frequency slightly lower than the resonant frequency of the l4, resulting from feedback through the small capacity between the anode and controlgrid of the pentode ,l;0. According to the invention, this component will be offset by a positive resistive component resulting from feedback of the drop across resistor l3 through the much larger capacity between screen and control grid. The drop across the resistor l3 arises in part from the flow of the modulated space current through it and through the stray capacities shunting it, and in part from that portion of the circulating current of circuit 14 which flows through it in series with the capacity of the anode to the suppressor grid and cathode of the pentode Ill. The component of the drop across resistor l3 which is in quadrature to the space current produces the compensatory positive resistive component of input impedance seen at the terminals H, while the inphase component merely increases the apparent capacity seen across the terminals. Since therphase of the second named circulating part of the current through the resistor l3 varies with frequency, the positive resistive component of impedance induced at the input terminals H also varies with frequency. It has been experimentally determined that the net resistive component resulting from the two. feedback links may be made negligible over the normal useful bandwidth of circuit M by proper choic of the values of the circuit elements such that substantially equal and opposite resistive components of impedance are induced across input terminals 1 I. In a typical amplifier operating at a frequency of about 460 kilocycles it has been found that optimum conditions require the resistor I3 to be of the order of ohms.

In Fig. 2 there is illustrated an alternative arrangement of the Fig. 1 embodiment for use in circuits wherein it is desired to operate the screen electrode at a considerably lower voltage than that supplied to the anode circuit. Here the resistor is connected between the screen and the positive terminal of the source l2 acts as a series dropping resistor and the condenser l6 connected between the junction of the resistor l3 and the circuit I4 and the screen electrode provides a path to the screen for the feedback balancing signal.

What is claimed is:

1. In an amplifier of wave signal energy comprising a vacuum tube having at least a cathode, a control electrode, a screen electrode and an anode in the order as named; a source of screen and anode voltage for said tube, a tuned load and a resistor serially connected between said anode and said source; said screen electrode being connected to the junction of said tuned load and said resistor so that said resistor is serially connected between said screen electrode and said source; and said source and resistor providing the path of lowest impedance between said screen grid and said cathode.

2. In an amplifier of wave signal energy comprising a vacuum tube having at least a cathode,

a control electrode, a screen electrode and an.

ponent of the input impedance thereof due to capacitive feedback to said control electrode from said anode; and said source and resistor providing the path of lowest impedance between said screen grid and said cathode.

GEORGE D. WOFFORD.

REFERENCES CITED 6 The following references are of record in the file of this patent:

anode in the order named; a source of screen and anode voltage for said tube; a tuned load and a resistor serially connected between said anode and said source; said screen electrode being connected to the junction of said tuned load and said resistor so that said resistor is serially connected between said screen electrode and said source; the value of said resistor being so selected that the voltage drop across it will reflect across the input of said amplifier a re sistive component of impedance of such magnitude and sense as :to neutralize the resistive com- UNITED STATES PATENTS Soller Jan. 8, 1946 

